Which class is more needed?
Both class are needed, in 4 man content PLD and WAR are interchangeable. In 8 man content most group will want 1 of each to provide their respective debuff and also so that they won't get hit by the Limit Break penalty. Some group may want Double PLD for Hallowed Ground if they are newer to that content.
Which class is preferable?
Like I said both classes are interchangeable, and work equally as well. PLD negates more damage due to Shield Oath, but WAR has more HP and is healed for more due to defiance. PLD also has a few more cooldowns then WAR, but WAR gets Inner beast which is more or less a mini heal + defense cooldown in one, very useful. WAR are also alot better at AOE tanking, Steel Cyclone and Overpower is amazing for AOE hate.
Which class is least played?
Before the WAR buff, everyone was a PLD. With the buff, I have been seeing alot of people leveling WAR. In endgame however they are pretty equal.
Warrior or Paladin?
Just play the one you enjoy more, I have both PLD and WAR at ilevel 90 and use them both. WAR is a little more fun though cause they have more combos, for PLD you will be just spamming Rage of Halone combo 90% of the time. If you are in a raid group you might also want to see what class is the other Tank, if he is a WAR, you be the PLD and vice versa.